Charged Particle Pseudorapidity Distributions Measured with the STAR EPD
Round 1
Reviewer 1 Report
This paper is very well written, clear, easy to read, and quite interesting. It deals with the measurement of the pseudorapidity distributions performed with the STAR EPD detector. My opinion is that it should be published as it is.
Author Response
Thank you for the evaluation of our manuscript and the positive opinion!
Reviewer 2 Report
The paper described several technological points studied for a newly constructed multiplicity detector in STAR for its Beam Energy Scan II (BES II). The new detector arrays are made of two sets of East and West arms, each consisting of 12 times 1000 arrays covering between 0.7 to 13.5 degrees.
The author described many effects on this detector array; multi-particle impact on individual detector elements, results from particles such as lambda, lepton, gamma, and nuclear fragments, and anticipated broadening etc. Finally, the author summarized all systematic uncertainty sources and many other numerical contributions, all of which are less than 6-8%
The description is clear, but I have a few questions.
#1 The author described that this new device enhanced the capabilities of centrality determination. However, the reader needs help understanding any disadvantages of the previous detector array. In other words, why this new detector array must be added or installed for the BES II experiment still needs to be clarified. This new device's motivation and requirement must be spelled much more clearly.
#2 Is your device better than the detector in PHOBOS? Or is there a granularity difference between the two? Describe more quantitatively the difference between the two detectors when you compare your data with the PHOBOS data.
#3 Line 24: Tracker At RHIC (STAR) must be Tracker at RHIC (STAR)
After these corrections, the article can be published in MDPI.
Author Response
The paper described several technological points studied for a newly constructed multiplicity detector in STAR for its Beam Energy Scan II (BES II). The new detector arrays are made of two sets of East and West arms, each consisting of 12 times 1000 arrays covering between 0.7 to 13.5 degrees.
The author described many effects on this detector array; multi-particle impact on individual detector elements, results from particles such as lambda, lepton, gamma, and nuclear fragments, and anticipated broadening etc. Finally, the author summarized all systematic uncertainty sources and many other numerical contributions, all of which are less than 6-8%
A: Thank you for the careful evaluation of the manuscript; I corrected everything according to your suggestions.
The description is clear, but I have a few questions.
#1 The author described that this new device enhanced the capabilities of centrality determination. However, the reader needs help understanding any disadvantages of the previous detector array. In other words, why this new detector array must be added or installed for the BES II experiment still needs to be clarified. This new device's motivation and requirement must be spelled much more clearly.
A: Thank you for the comment! The following was added after line 34:
“The EPD is a completely new subdetector that was supposed to improve the event plane resolution: for example, by about a factor of 2 in Au+Au collisions at $\sqrt{s_{\text{NN}}}=19.6$ GeV~\cite{tlusty2018rhic}. Its predecessor (in event plane determination), the Beam-Beam Counter (BBC) has much less fine granularity than the EPD: only 36 tiles, with the 18 inner smaller tiles used \-- compared to the 372 tiles of the EPD~\cite{starepd}. It also has smaller acceptance of $3.3<|\eta|<5.0$ in pseudorapidity~\cite{bbc_10.1063/1.2888113}.”
citing the proper sources.
#2 Is your device better than the detector in PHOBOS? Or is there a granularity difference between the two? Describe more quantitatively the difference between the two detectors when you compare your data with the PHOBOS data.
A: Thank you for the question! It is hard to tell which device is better; in fact, they are very different detectors. In any case, the paragraph after line 233 was expanded as follows:
“Another experiment of the RHIC complex was the PHOBOS experiment, which completed data taking in 2006. The PHOBOS was a large acceptance silicon detector, covering almost $2\pi$ in azimuth and $|\eta|<5.4$ in pseudorapidity~\cite{phobos_fragments}. Compared to STAR’s EPD, there are differences in both detector topology and granularity: the silicone pad detectors measure the total number of charged particles emitted in the collision, with modules mounted onto a centrally located octagonal frame (Octagon) covering $|eta|\leq 3.2$, as well as three annular frames (Rings) on either side of the collision vertex, extending the coverage out to $|\eta|\leq 5.4$~\cite{back2005phobos}.
The PHOBOS also measured $\der N_{\text{ch}}/\der\eta$ at 19.6, 62.4, 130, 200 GeV energies~\cite{phobos}. Although in that paper a slightly different centrality binning was used (0--3\%, 3--6\% and 6--10\% instead of 0--5\% and 5--10\%; the other centrality classes were the same), at 19.6 GeV the results can be compared.”
citing the proper sources.
#3 Line 24: Tracker At RHIC (STAR) must be Tracker at RHIC (STAR)
A: Thank you, corrected!
After these corrections, the article can be published in MDPI.
A: Please note that at the end, additionally, the following was added to the Discussion:
“Measuring pseudorapidity values of charged particles is important due to the possibility of estimating the initial energy density of the quark--gluon plasma created in the collisions, based on them.~\cite{Csanad:2016add,Ze-Fang:2017ppe}"
to put the measurement a bit more into perspective.
Reviewer 3 Report
In this contribution to the special issue on the Zimányi School – Heavy Ion Physics, the author presents on behalf of the STAR collaboration a measurement of the charged particle pseudorapidity density, using the recently-installed Event Plane Detector. Using Monte Carlo simulations in conjunction with an iterative unfolding procedure, it was possible to characterize the detector response matrix, which allows to extract the underlying particle distribution. A number of sources of systematic error were investigated and included in the reported uncertainty. Despite the careful systematic checks, agreement with previous measurements from the PHOBOS collaboration is poor. Nevertheless, the analysis is careful and comprehensive, and described clearly. Further, both these results and future results that can be obtained with this method are of significant interest. As such, I recommend publication.
Author Response
Thank you for your thorough review, and also for the positive evaluation!
Round 2
Reviewer 2 Report
The revision seems fine to me, so please accept the paper.